In these days, a display device for visually recognizing information processed in information processing device has been rapidly developed according as information-processing devices develop.
The display device is an interface to change data having an electrical signal format into data having an image signal format, thereby to be recognized by users. The display device may be classified into various types according to a driving method thereof.
The display device is classified into a CRT (Cathode Ray Tube) type display device having an analog driving method, an LCD (Liquid Crystal Display) device having a digital driving method and an organic EL display device that has been recently developing.
The CRT type display device has a disadvantage such as the size and weight of the CRT type display device increase in proportion to a display size of the CRT type display device. The LCD device has an advantage such as the size and weight of the LCD device do not increase although the display size of the LCD device increases. The LCD device displays images by controlling a transmissivity of a light passing through a liquid crystal.
The organic EL device displays the image by means of an organic EL material disposed between two electrodes. The organic EL material emits lights when a forward current is applied between the two electrodes like a diode.
There are weight and size differences between the LCD device and the organic EL device because of the differences of operation properties between the LCD device and the organic EL device.
Particularly, the LCD device requires a backlight assembly for increasing a uniformity of the light used to display the image. However, the organic EL device does not require a light source, such as the backlight assembly, because the organic EL material emits lights of its own accord, thereby reducing the weight and size of the display device.
FIG. 1 is a plan view showing a conventional organic EL device. FIG. 2 is a cross-sectional view cut along a line of II–II for showing a structure of the conventional organic EL device shown in FIG. 1.
Referring to FIGS. 1 and 2, the conventional organic EL device includes a transparent glass substrate 10 and TFTs 20 disposed on the transparent glass substrate 10 in a matrix configuration. The TFTs 20 are formed by a semiconductor thin film process. The TFTs 20 includes gate electrode, source electrode, drain electrode and channel layer (not shown).
The gate electrodes of TFTs arranged in a same column between the TFTs 20 in the matrix configuration are commonly connected with a gate line (not shown).
The source electrodes of TFTs arranged in a same row among the TFTs 20 in the matrix configuration are commonly connected with a data line (not shown).
The drain electrodes of the TFTs 20 in the matrix configuration are respectively connected with anode electrode 30 made of an ITO (Indium Tin Oxide) material. The anode electrode 30 supplies a hole.
A red organic EL layer 40 for emitting a light having a red wavelength, a green organic EL layer 50 for emitting a light having a green wavelength and a blue organic EL layer 60 for emitting a light having a blue wavelength is disposed on each of the anode electrodes 30, respectively.
In order to emit the red, green, and blue light, the red, green and blue organic EL layers 40, 50 and 60 require a cathode electrode 70 for supplying electrons and an anode electrode 30 for supplying holes.
The cathode electrode 70 is made of aluminum or an aluminum alloy. The cathode electrode 70 is disposed with a uniform thickness on a whole surface of the transparent glass substrate 10 to cover the red, green and blue organic EL layers 40, 50 and 60.
The cathode electrode 70 receives a cathode power source through only one external power supply line 80. The red, green and blue organic EL layers 40, 50 and 60 receive the cathode power source having a same voltage level from the cathode electrode 80.
The organic EL device may display images, moving pictures and characters, by applying a driving signal, which is appropriate to display the image, to each of the anode electrodes 30.
However, even though the same forward current is commonly applied to the red, the green and the blue organic EL layers 40, 50 and 60, it is difficult to precisely display images in a full-color and with a high resolution because brightness from the red, green and blue organic EL layers 40, 50 and 60 is different to each other.